1. Field of the Invention
The present invention relates to a high velocity electron beam scanning negatively charge biased photoconductive image pickup tube in which a photoelectric signal is read by scanning a target with a high velocity electron beam.
2. Description of the Prior Art
The high velocity electron beam scanning negatively charge biased image pickup tube has been known for a long time as possessing, in principle, the advantage that the capacitive lag is negligible owing to a low beam resistance and that substantially no beam bending is involved. Thus, image pickup tube characteristics unattainable in the past can be expected. Examples of the image pickup tube of the specified type are contained in the official gazette of Japanese Laid-open Patent Application No. 54-44487, an article by J. Dressner; RCA Review, June (1961), pp. 305-324, "High Beam Velocity Vidicon", etc. Since, however, a photoconductive material meeting the aforementioned merits has not been found as yet, the image pickup tube has not been put into practical use.
On the other hand, it has been known that amorphous silicon containing hydrogen (hereinbelow, abbreviated to "a-Si:H") exhibits a high photoelectric conversion efficiency. A photoconductive image pickup tube employing an a-Si:H photoconductive layer has already been proposed. Examples of such image pickup tube are contained in U.S. Pat. No. 4,255,686 and British Pat. No. 1,349,351.
All the examples, however, concern image pickup tubes of the low velocity electron beam scanning type (hereinbelow, termed "LP operation mode").
When an amorphous silicon layer is used as a photoconductive layer in the image pickup tube of the LP operation mode, the characteristics of the tube are subject to such limitations as (1) inferior lag characteristics, (2) inferior photo-response for light of shorter wavelength, and (3) a distorted picture ascribable to the bending of the scanning electron beam.
It has accordingly been impossible to expect sharp enhancements in the characteristics of the device under the current circumstances.